1. Field of the Invention
The present invention relates to a laser processing head for processing a to-be-processed work with a laser beam by focusing the laser beam through a condensing optical system and projecting the beam through a processing nozzle while injecting an assist gas through the processing nozzle.
2. Description of the Related Art
In a laser processing head of a laser machine, a condensing optical system, for focusing a laser beam, is held and an assist gas feed port for feeding an assist gas into the laser processing bead is formed. The laser beam focused through the condensing optical system, such as a condensing lens, and the assist gas fed through the assist gas feed port, are directed to the to-be-machined work though the processing nozzle of the laser processing head so that the work is processed. In the laser processing head, further, the space between the condensing optical system and the processing nozzle is called a laser processing head chamber.
In recent years, further, a carbon dioxide laser has been used as a high-output laser. In the carbon dioxide laser, it becomes necessary to decrease the number of condensing optical systems such as the number of the transparent condensing lenses as much as possible from the standpoint of the wavelength. Therefore, the laser processing head holds a single condensing optics. Atmospheric pressure exists in the space on the side opposite to the laser processing head chamber relative to the condensing lens. The laser processing head and the condensing lens can be moved relative to each other by an adjusting mechanism to freely adjust a positional relationship between the processing nozzle and the condensing lens.
During processing with the laser beam, the assist gas is fed into the laser processing head from the assist gas feed port, and the interior of the laser processing head chamber achieves a predetermined high pressure of, for example, 1 MPa. The condensing lens is held by a condensing lens-holding portion, which is larger than the condensing lens and surrounds it. When the condensing lens-holding portion has an outer diameter of, for example, 60 mm, a force which is as great as about 2.8 kN is exerted on a pressure-receiving surface constituted by the condensing lens and by the condensing lens-holding portion. Against this force, the adjusting mechanism must move the processing nozzle and/or the condensing lens relatively. Therefore, a large drive unit is necessary for operating the adjusting mechanism so that the machining head itself becomes bulky or complex.
To solve this problem, Japanese Patent Publication No. 2804206 discloses a laser processing head equipped with an annular gas sub-chamber and a piston cylinder. FIG. 8 is a sectional view in the longitudinal direction of a conventional laser processing head as disclosed in the Japanese Patent Publication No. 2804206. The laser processing head 114 disclosed in FIG. 8 is mainly constituted by a nozzle portion 105 in which a nozzle hole 111 is formed, and a condensing lens-holding portion 103 for holding a condensing lens 102. A laser processing head chamber 108 is formed between the condensing lens 102 and the nozzle hole 111.
As shown, an upper annular protrusion 117 and a lower annular protrusion 109 are protruded from the inner peripheral surface of the nozzle portion 105. The outer peripheral surface of the condensing lens-holding portion 103 is engaged with the upper annular protrusion 117 and with the lower annular protrusion 109. Further, the condensing lens-holding portion 103 is coupled, by a coupling portion 107, to a piston 106a of an actuator 106. Due to the actuator 106 which is the adjusting mechanism, the condensing lens-holding portion 103 can slide in the nozzle portion 105.
An assist gas feed port 113 is formed in the nozzle portion 105 so as to be communicated with the laser processing head chamber 108. The assist gas is fed from the source of the assist gas disposed on the outer side of the laser processing head 114 into the laser processing head chamber 108 through a feed passage 119 and the assist gas feed port 113. The assist gas is fed at all times during the processing with the laser beam, and a predetermined high pressure is maintained in the machining head chamber 108.
As shown, a flange 116 arranged between the upper annular protrusion 117 and the lower annular protrusion 109 is provided on the condensing lens-holding portion 103. The end of the flange 116 is sized to engage with the inner peripheral surface of the nozzle portion 105. A gap between the condensing lens-holding portion 103 and the nozzle portion 105 is sealed with a sealing member. Therefore, an annular gas sub-chamber 115 is formed between the upper end surface of the flange 116 and the lower end surface of the upper annular protrusion 117.
An opening portion 153 is formed in the lower end surface of the upper annular protrusion 117. A branch passage 142 branched from a branching portion 141 in the feed passage 119 extends to the opening portion 153. Therefore, the assist gas in the feed passage 119 is fed into the annular gas sub-chamber 115 through the branch passage 142.
As shown, if the inner diameter of the lower annular protrusion 109 is denoted by D1, the inner diameter of the nozzle portion 105 by D2 and the inner diameter of the upper annular protrusion 117 by D3, the pressure-receiving area of the condensing lens-holding portion 103 is expressed by DI2/4×π and the pressure-receiving area of the annular gas sub-chamber 115 is expressed by (D32−D22)/4×π. In FIG. 8, the pressure-receiving area of the condensing lens-holding portion 103 is formed to be equal to the pressure-receiving area of the annular gas sub-chamber 115. Therefore, a force which the condensing lens-holding portion 103 receives is offset by a force which the annular gas sub-chamber 115 receives. Accordingly, the force required for maintain the condensing lens-holding portion 103 at the present position by the actuator 106 or for moving it under a condition pressurized with the assist gas by the actuator 106 is relatively small.
When the processing with a laser beam ends, the assist gas is no longer fed from the source of assist gas. The assist gas in the laser processing head chamber 108 is released from the nozzle hole 111, and the pressure in the laser processing head chamber 108 gradually decreases. Similarly, the assist gas in the annular gas sub-chamber 115 flows into the laser processing head chamber 108 through the branch passage 142 and the feed passage 119 and is similarly released from the nozzle hole 111.
However, the laser processing head 114 of the prior art as disclosed in the Japanese Patent Publication No. 2804206 is provided with the annular gas sub-chamber 115 and necessitates the branch passage 142 for feeding the assist gas or the compressed air into the annular gas sub-chamber 115. Therefore, the structure of the laser processing head 114 as a whole becomes complex. When the annular gas sub-chamber 115 is provided, further, it becomes difficult to maintain a balance of pressure between the laser processing head chamber 108 and the annular gas sub-chamber 115.
Further, when the processing with the laser beam ends and-the pressure decreases in the laser processing head chamber 108, it becomes necessary to pass the assist gas in the annular gas sub-chamber 115 into the laser processing head chamber 108 through the branch passage 142 and the feed passage 119. Therefore, a considerable period of time is necessary for decreasing the pressure in the annular gas sub-chamber 115.
Further, when the type of processing of the laser-machine is to be changed, the kind of assist gas is often changed, e.g., an assist gas which is oxygen is changed to nitrogen. In changing the assist gas, the gas remaining in the annular gas sub-chamber 115 must be released. Similarly, therefore, a considerable period of time is required for changing the assist gas.
The present invention was accomplished in view of the above circumstances, and has an object of providing a laser processing head which enables the condensing optical system-holding means or the nozzle-holding means to be moved with a small force without providing an annular gas sub-chamber and/or a branch passage for the annular gas sub-chamber.